Government plans to save around £2.5bn a year by raising the retirement age to 66 will hit women hardest, according to an independent thinktank that has described the move as unfair.

It warned that women will be affected most by the plans to make workers wait a year longer before receiving the state pension, especially as the majority of them are already out of the workforce by the time they reach their 60s and are unlikely to find work again. The Pensions Policy Institute, which rarely criticises government policy, said ministers were giving people nearing retirement "insufficient notice".

A row over the cost-saving plan, which is due to come into effect from 2016, has simmered since it was put forward last year by pensions minister Steve Webb. Pensioner groups have lobbied ministers and individual Tory and Liberal Democrat MPs to delay the measure, which is seen by the government as part of an overhaul of Britain's costly benefits system.

The row is expected to boil over next week when Webb appears before MPs on the work and pensions select committee. Webb, who was Liberal Democrat pensions spokesman before joining the coalition, is expected to come under attack for hitting poorer pensioners unable to work beyond the current retirement age.

The PPI said: "In 2010, around 76% of men aged between 55-59 [were] still economically active; by age 60-64 that figure drops to 54%. Once individuals have left the labour market at older ages it may be difficult for them to re-enter it."

It argued that women were badly affected by the changes as only 34% were economically active after reaching 60. The state pension age for women has been gradually increasing since 6 April last year and was intended to reach 65 by 2020. Both men and women's pension age would then increase to 66 by 2026. But the chancellor scrapped those plans in October and now the increase will be accelerated to equalise the pension age for men and women at 66 by 6 April 2020.

The effect is that 500,000 women must work at least a year longer than they would have under the previous acceleration, with 300,000 of them working an extra 18 months and 33,000 born between 6 March and 5 April 1954 working two extra years.

Webb insisted reform was necessary at a time of increasing life expectancy. He said: "In a country where 10 million of us will live to be 100, we simply can't go on paying the state pension at an age that was set early in the last century.

"Although women will experience the rise in the state pension age more quickly than previously planned, they will still draw the state pension for an average of 23 years. Our 'triple guarantee' means someone retiring today on a full basic state pension will receive £15,000 more over their retirement than they would have done under the old prices link."